In recent years, there has been advanced a trend towards a shorter wavelength of a laser light source used as a light source for reproducing of information recorded on an optical disc and for recording information on an optical disc, and a laser light source with a wavelength of 405 nm such as, for example, a violet semiconductor laser or a violet SHG laser that conducts wavelength conversion of an infrared semiconductor laser by utilizing second harmonic generation has been put to practical use.
When these violet laser light sources are used, information in volume of 15-20 GB can be recorded on an optical disc having a diameter of 12 cm, in the case of using an objective lens having a numerical aperture (NA) identical to that of a digital versatile disc (hereinafter referred to as DVD for short), and information in volume of 23-27 GB can be recorded on an optical disc having a diameter of 12 cm, in the case of enhancing NA of the objective lens to 0.85. Hereafter, an optical disc employing a violet laser light source and a magnetic optical disc are called “high density optical disc” generically in the present specification.
Incidentally, as a high density optical disc, there are proposed two standards at present. One of them is Blu-Ray disc (hereinafter referred to as BD for short) that uses an objective lens with NA 0.85 and has a protective layer whose thickness is 0.1 mm, and the other is HD DVD (hereinafter referred to as HD for short) that uses an objective lens with NA 0.65-0.67 and has a protective layer whose thickness is 0.6 mm. When considering possibility of circulation of these two high density optical discs in the two standards on the market, compatible optical pickup apparatuses which can conduct recording and/or reproducing for not only existing DVD and CD but also for all types of high density optical discs are important, and a one-lens system that is compatible by using a common objective lens for a high density disc, DVD and CD amount them is most ideal.
With respect to an optical pickup apparatus having compatibility for plural types of optical discs, there have been known technologies to change a degree of divergence for a light flux entering an objective optical system or to provide a diffractive structure on an optical surface of an optical element constituting an optical pickup apparatus, as a method of correcting spherical aberration caused by differences in a light flux wavelength and a protective layer thickness used in respective optical discs (for example, see Patent Document 1).
However, when achieving compatibility for BD, HD DVD and CD, coma of an objective lens caused by its tracking is great, and preferable recording signals and reproducing signals cannot be obtained accordingly, when trying to achieve compatibility only by a method to change a degree of divergence of a light flux entering an objective optical system (hereinafter referred to as an objective lens), because a difference of substrate between respective optical discs is great. On the other hand, when trying to achieve compatibility by providing a diffractive structure on an optical element and by utilizing diffracting effects, high amount of light of 90% or more cannot be obtained because of a wavelength ratio of violet laser light used for high density disc to infrared laser light used for CD. Further, since wavelength dependency of the diffractive structure is determined by the design for achieving compatibility, if a wavelength of each incident light entering an objective lens is varied or changed, wavefront aberration is affected.    Patent Document 1: TOKUKAI No. 2002-298422
The invention described in Patent Document 1 is one to record and reproduce information for respective optical discs by making optical system magnification of the objective lens for each of plural types of optical discs to be 0. In this method, however, a change in wavelength characteristic caused by diffracting effects grows greater, and wavefront aberration is worsened, when an oscillation wavelength is varied among laser lots, although coma is hardly generated by lens shifting in the course of tracking, which is a problem. Further, there is a problem that an objective lens seems most likely to be designed giving top priority to the high density disc, and light utilization efficiency for the lowest density optical disc is insufficient, although light utilization efficiency for CD representing an optical disc of lowest density is improved. Therefore, it is difficult to realize higher speed of recording and reproducing information for the lowest density optical disc which is more specifically CD, which is a problem.
Further, when the technology disclosed in the Patent Document 1 is applied as it is, there are problems that an increase of manufacturing cost for optical pickup apparatuses and worsening efficiency of manufacturing operations are caused.